1. Field of the Invention
The present invention generally relates to liquid crystal display devices, and more particularly, to a liquid crystal display device adapted to be driven by a television signal.
2. Description of the Prior Art
FIG. 19 of the accompanying drawings illustrates, in the form of an equivalent circuit, the typical prior art liquid crystal display device. Reference to FIG. 19 will now be described for the purpose of discussing of the prior art which is believed to be pertinent to the present invention.
The prior art liquid crystal display device shown in FIG. 19 comprises a plurality of liquid crystal cells 1, each shown in the form of an equivalent capacitor, its display electrodes 2 and its mating counter electrodes 3. A thin film transistor 4 of MIS structure is connected to each liquid crystal cell 1 as a switching active element for driving the respective liquid crystal cell 1. In practice, the liquid crystal cells 1 and the thin film transistors 4 are arranged in a matrix having a plurality of, for example, four, rows of source lines S1, S2, S3 and S4 and a plurality of, for example, four, columns of control lines G1, G2, G3 and G4, only a portion of which is shown in FIG. 19. As shown, the transistors 4 in each of the first through fourth rows have their gates connected to the associated control line G1, G2, G3 or G4. The first through fourth control lines are adapted to be scanned according to an interlaced scanning scheme so that the first and third control lines G1 and G3 can be excited by odd-numbered line scanning of the television signal whereas the second and fourth control lines G2 and G4 can be excited by even-numbered line scanning of the same television signal. A predetermined voltage inverted for each frame is applied to the counter electrodes of the respective liquid crystal cells 1 as a drive voltage.
Video signals (for example, R, G and B signals) to be displayed are supplied through the source lines S1 to S4.
During the odd-numbered field of the television signal, the first and third control lines are successively excited by a scanning signal (line sequence pulses conforming to a horizontal synchronizing signal) so that a video signal voltage can be sequentially applied to the liquid crystal cells in the first row and the liquid crystal cells in the third row. At this time, the transistors in each of the second and fourth rows are switched off.
During the even-numbered field of the television signal, the second and fourth rows are sequentially scanned with the consequence that the video signal is applied to the liquid crystal cells in the second and fourth rows. At this time, the transistors in each of the first and third rows are switched off and the signal in the odd-numbered field is retained.
During the subsequent frame, the polarity of the voltage applied between the electrodes 2 and 3 of the liquid crystal cells 1 is reversed. In such case, the liquid crystal drive frequency will be 15 Hz if a television signal according to NTSC system is used to drive the liquid crystal cells. This is partly because the television signal is based on the interlaced scanning scheme in which the odd-numbered and even-numbered horizontal lines of the screen are scanned for each different field and partly because the electric field applied to the liquid crystal cells 1 is required to be cyclically reversed during the liquid crystal cells' lifetime. Considering one picture element for facilitating a better understanding, the application of a positive voltage between both electrodes of each cell is repeated at intervals of four fields. This is because the picture element referred to above is, after having been scanned at a n-th field, not scanned during the next succeeding field, the (n+1)th field for the purpose of interlacing; but will be scanned at the subsequent (n+2)th field while being applied with a negative voltage during the lifetime of the liquid crystal cells; and will not be scanned during the next succeeding field, (n+3)th field, for the purpose of interlacing. At the (n+4)th field, the liquid crystal cell of the picture element is again applied with a positive voltage. Thus, the application of the positive voltage to each liquid crystal cell takes place at intervals of the four fields which, in terms of the drive frequency, correspond to 15 Hz according to the NTSC system or 12.5 Hz according to the PAL system.
In the prior art liquid crystal display device of the above discussed construction, in view of the fact that the liquid crystal cells are alternately driven, the liquid crystal drive frequency will become one half the frame frequency, that is, 15 Hz, when display is effected based on the interlaced scanning scheme. While the frequency of 30 Hz will not be perceived as a flicker by human eyes, the frequency of 15 Hz is recognized as a flicker appearing on the screen and, as a result, a picture uncomfortable to look will be reproduced. In order to substantially avoid this problem, a drive method has been suggested wherein the combination of the two rows is changed for each field so that every two rows can be simultaneously driven by the same video signal. According to this suggested method, although the drive frequency can be improved to 30 Hz, no improvement has been made in the number of effective display scanning lines per field. Specifically, assuming that the number of the effective display scanning lines driven by the same signal is 480 lines, the number of the effective display scanning lines per field remains 240 lines and, accordingly, the vertical resolution is still insufficient.
Another method suggested to substantially avoid the above discussed problem is that the use is made of a frame memory so that an image data corresponding to two video signal lines can be displayed during the scanning period of one horizontal scanning line Gi (i=1 to m). However, this method has problems in that the use of a memory having a large memory capacity corresponding to the number of the display liquid crystal cells c is required and that a high speed is required in the clock frequency for a drive driver to drive the active elements, resulting in the increased manufacturing cost.